1. Field of the Invention
The present invention relates to an optical information reproducing method of reproducing multi-level information recorded on an information recording medium, such as an optical disk, and an apparatus for the method, and more particularly, to a method of efficiently performing level correction and automatic gain control (AGC) on a reproduction signal with high precision, and an apparatus for the method.
2. Description of the Related Art
Up to now, binary digital data is recorded on a spiral or concentric track of an optical disk. As recording methods, there are recording methods based on concave and convex pits formed by embossing, or the like (for a ROM disk), holes formed in an inorganic or organic recording film (for a write-once disk), a crystal state difference (for a phase change disk), a magnetization direction difference (for a magneto-optical disk), and the like.
When the recorded data is to be reproduced, a laser beam is emitted to the track to detect an intensity difference between reflected light beams, a polarization direction difference caused by a magnetic Kerr effect, and the like, thereby obtaining a reproduction RF signal. Then, binary data is detected based on the obtained reproduction RF signal.
In recent years, research and development for increasing a recording density of the optical disk are under way. A technique for more efficiently performing multi-level recording and reproduction using light spots having a same size has been proposed as an information recording and reproduction technique using a very small light spot.
For example, the inventors of the present invention proposed a method based on a multi-level recording and reproducing technique in Japanese Patent Application Laid-Open No. H05-128530. That is, according to the method, multi-level information is recorded on an information track of an optical information recording medium based on a combination of a width of an information pit in a track direction and an amount of shift of the information pit in the track direction relative to a reproduction light spot. In Japanese Patent Application Laid-Open No. H05-128530, there is also proposed a method of reproducing multi-level information based on a correlation between a detection signal obtained by learning in advance and a detection signal obtained from a light spot when the recorded multi-level information is to be reproduced from the information pit.
Further, a recording method of recording multi-level information (cipher method) based on the amount of shift of the information pit in the track direction has been proposed.
Here, a reproducing method in a case of eight-level recording in which the width of the information pit in the track direction is changed stepwise will be described. First, when information is to be recorded on the information recording medium, an eight-level information pit is formed on each cell after the information is converted from binary data into one of eight levels.
In the case of the eight-level recording, each cell corresponds to binary data of three bits. For example, as shown in FIG. 20, with respect to information of three bits, (0, 0, 0) corresponds to a level 0, (0, 0, 1) corresponds to a level 1, (0, 1, 0) corresponds to a level 2, and (0, 1, 1) corresponds to a level 3. In addition, (1, 0, 0) corresponds to a level 4, (1, 0, 1) corresponds to a level 5, (1, 1, 0) corresponds to a level 6, and (1, 1, 1) corresponds to a level 7.
For example, as shown in FIG. 20, in order to select one of the eight-level information pits, a width of a cell in the track direction is divided into sixteen. Assume that the level 0 indicates that no information pit for recording is formed, the level 1 indicates 2/16 of the width of the cell, and the level 2 indicates 4/16 of the width of the cell. In addition, assume that the level 3 indicates 6/16 of the width of the cell, the level 4 indicates 8/16 of the width of the cell, the level 5 indicates 10/16 of the width of the cell, the level 6 indicates 12/16 of the width of the cell, and the level 7 indicates 14/16 of the width of the cell.
The selected information pit for recording is formed at random. A reproduction signal corresponding to the amount of light which is reflected on the multi-level information pit and received by a photodetector is sampled at a timing when a center of a light spot moves to a center of the width of the cell in the track direction. As a result, as shown in FIG. 21, amplitude distributions of reproduction signals corresponding to respective levels are obtained.
In FIG. 21, when the level 0, in which no information pit for writing is formed, is repeated, the amplitude of the reproduction signal is normalized as “1”. In addition, when the information pit for recording, which corresponds to the level 7, is repeated, the amplitude of the reproduction signal is normalized as “0”.
A value of the reproduction signal corresponding to each of the levels has a width because of an influence of the information pits formed before and after a target information pit (inter-symbol interference). As is apparent from FIG. 21, when the amplitude distributions of the reproduction signals corresponding to adjacent levels overlap with each other, the levels cannot be separately detected based on a fixed threshold value.
In order to solve this problem, the following method is described in a report presented in an ISOM 2003 meeting (see ISOM 2003 meeting “Write-once Disks for Multi-Level Optical Recording”, Technical Digest Fr-Po-04). That is, according to the described method, reproducing signals are read from a series of pits in which a value of a target information pit and values of information pits before and after the target information pit are known in advance and then stored (learning). A value of the reproduction signal from an actual information pit is compared with the stored values (correlating) to separately detect the levels.
The above-mentioned methods are examples of the conventional multi-level recording and reproduction. In any of the methods, because of various factors, such as a reflectance difference between various optical disks in an optical disk and a reproduction frequency characteristic difference between an inner circumference side and an outer circumference side in a single optical disk, a variation in level or amplitude of the reproduction signal occurs. In addition, there is an influence caused by a mechanical factor difference or an optical factor difference between optical disk drives. Therefore, it is necessary to eliminate those factors.
According to a method of suppressing the level variation or the amplitude variation in the case where multi-information is to be reproduced, in general, fixed pattern regions, each of which is located between adjacent data regions, are provided at predetermined intervals to perform the automatic gain control (AGC) or level correction.
FIG. 22 is a simple diagram showing a normal block structure of the regions. As shown in FIG. 22, the fixed pattern region and the data region of the multi-level information are alternately recorded on the optical disk. The fixed pattern region is composed of a bias mark, a gain mark, and a clock mark.
A function of each of the marks will be described. In a case of the bias mark, a minimum mark is recorded (or no mark is recorded) on a cell to detect a minimum signal level. A level of a reproduction signal from a data region is corrected based on the minimum signal level as a reference level.
In a case of the gain mark, a maximum mark is recorded on a cell to detect a maximum signal level. The AGC is performed on a reproduction signal from a data region based on the maximum signal level. In a case of the clock mark, a tone signal is recorded on a cell to perform zero cross detection. A phase deviation from a clock synchronized with multi-level data is detected based on the tone signal, thereby performing a PLL operation.
When clock marks are used, sampling timings of subsequent data regions can be made to coincide with one another. In some cases, the fixed pattern region further includes address information and waveform equalization adaptive information, which are omitted in FIG. 22.
When a density is to be further increased, a size of the cell relative to the light spot becomes increasingly smaller, so a new problem occurs in the case where the level correction and the AGC are performed. That is, an S/N ratio of the reproduction signal is reduced by the inter-symbol interference with adjacent cells, with a result that the reproduction signal is more easily influenced by the level variation or the amplitude variation, which is caused by not only a low frequency component, but also a noise having a frequency close to a reproduction signal frequency band.
Therefore, a reproduction error rate significantly increases. For example, in a case of multi-level information data whose reproduction signal frequency is 22 MHz, a low frequency component of 50 kHz or less can be removed by a high-pass filter, or the like. However, in a practical case, noises are generated by a wobble component of a substrate groove, a substrate noise, or the like, even in a frequency band of 1 MHz to 5 MHz. The noises cannot be easily separated from the reproduction signal component.
As a result, the reproduction signal level is influenced by the noises, so it is likely to generate an erroneous determination value. Those problems can be avoided by increasing the number of fixed pattern regions to increase frequencies of the level correction and the AGC. However, the number of data regions cannot be ensured by the increase in the number of fixed pattern regions. Therefore, there is a problem in that format efficiency reduces.